Study of Possible Frequency Dependence of Small AC Fields on Magnetic Flux Trapping in Niobium by Polarized Neutron Imaging

Reducing the size of ambient magnetic flux trapping during cooldown in superconducting radio-frequency niobium cavities is essential to reaching the lowest power dissipation as required for continuous wave application. Here, it is suggested that applying an alternating magnetic field superimposed to the external DC field can potentially reduce the size of trapped flux by supporting flux line movement. This hypothesis is tested for the first time systematically on a buffered chemically polished (BCP) niobium sample before and after high temperature annealing, a procedure which is known to reduce flux pinning. External low-frequency (Hz-range) magnetic fields were applied to the samples during their superconducting transition and the effect of varying their amplitude, frequency and offset was investigated. A few results can be highlighted: The influence of the frequency and magnitude of the AC fields on the flux trapping in the untreated Nb sample cannot be neglected. The trapped flux seems to be homogeneously distributed, unlike the flux trapping in, e.g., lead (Pb), which is a type I superconductor. After annealing, the Nb sample shows practically no dependency of flux trapping on external AC fields. The trapped magnetic flux was measured by polarized neutron imaging, and calculations of trapped fields show good agreement with experimental results.

AC versus DC field effects on the crystallization behavior of a molecular liquid, vinyl ethylene carbonate (VEC)

We show that high ac-fields can modify the crystallization patterns of glass-forming liquids and replicate the effects seen using dc-fields.

How a Potentially Novel Effect Converts AC Current into DC Heat Flow

A novel current-induced thermoelectric phenomenon, the nonlinear anomalous Ettingshausen effect, was discussed. As an example, the weak charge ordering state in an organic conductor α-(BEDT-TTF)2I3 was focused on. This effect generates a transverse heat current with rectifying characteristics, namely unidirectionality even under AC fields.